1. Field of the Invention
The present invention relates to an anti-corrosive composition for iron and steel surfaces. Especially the composition is intended to be used for protecting the reinforcing bars of concrete. More particularly, the composition is to be used for the reinforcing bars of autoclaved light-weight concrete. The expression "light-weight concrete" includes aerated light-weight concrete, that is, light-weight concrete obtained by incorporating a gas-developing agent such as aluminium powder in the light-weight concrete raw mixture to cause the latter to expand after casting and so-called foamed light-weight concrete, that is, light-weight concrete obtained by foaming the raw material mixture before casting it. The invention is especially concerned with a composition for the reinforcing steel bars of autoclaved light-weight aerated concrete (abbreviated as ALC hereinafter). That type of light-weight concrete has usually a specific gravity of 0,4 to 1,2. The invention is also concerned with a process for the manufacture of steam-cured light-weight aerated concrete.
2. Description of the Prior Art
ALC is produced by mixing silicious raw materials and calcarious raw materials with water, incorporating the resulting mix with aluminium powder and other additives to make a slurry, pouring the slurry in a mould containing reinforcing steel bars, permitting the slurry to expand and to become half-plastic, cutting the half-plastic concrete into a desired shape using a piano wire or the like, and finally curing the cut concrete with steam in an autoclave.
ALC thus produced is a building material which is light in weight and superior in heat insulation and durability. However, it has a disadvantage that it contains pores accounting for about 80% of its volume and the pores permit moisture and corrosive substance to migrate through ALC so that to corrode the reinforcing steel bars.
For anti-corrosion, it has been a common practice to apply a proper anti-corrosive mixture to reinforcing steel bars. Conventional anti-corrosive materials are divided into three groups. (1) One which contains cement as the major component, (2) One which contains asphalt as the major component, (3) One which contains resin emulsion or latex as the major component.
The anti-corrosive mixture of the first group is most widely used because it is inexpensive, it produces good bond to both ALC and steel bars (simply referred as "bond" hereinafter), and it is moderately satisfactory in anti-corrosiveness. On the other hand, it has some disadvantages. For example, it is limited in pot life; it is liable to the variation of cement quality; it permits water to migrate; it forms an anti-corrosive coating poor in flexibility; and it is not easily applied to reinforcing steel bars thinner than 5 mm in diameter which are used for thin ALC panels, because of its poor flexibility.
The anti-corrosive mixture of the second group covers generally the deficiency of the one of the first group. However, it is still very poor in bond.
The anti-corrosive mixture of the third group has an advantage of having a long pot life and stable quality. However, it greatly varies in flexibility, bond and anti-corrosive performance depending on the type of emulsion or latex and mixing ratio of emulsion or latex and other additives. According to information disclosed so far, the anti-corrosive mixture of the third group is composed of a synthetic rubber, thermoplastic resin or thermosetting resin, and inorganic powder; a rubber latex, silica powder and lime; or a styrene-butadiene rubber latex and an inorganic substance such as kaolin and slaked lime. This anti-corrosive mixture has the following disadvantages. It costs more than conventional ones containing cement as the major component although it provides thinner coating for anti-corrosion, because it contains resin or latex in a comparatively large portion. It provides not always sufficient bond and flexibility, and its other disadvantage is formation of cracks in the relatively thicker part of coating. Therefore it is difficult to apply this anti-corrosive mixture of certain composition commonly both to reinforcing steel bars thicker than 5 mm for thick panels and to steel bars or nets of for instance 1-3 mm in thickness for thin panels, since in general the thicker the bars are, the thicker the coating may be.
In addition to the above-mentioned three groups of anti-corrosive mixtures, there are many known anti-corrosive mixtures containing a resin emulsion or latex and cement as the major component. Cement used in combination with a resin emulsion or latex inevitably shortens the pot life. On the other hand, there is disclosed a method of forming an anti-corrosive coating by applying different kinds of anti-corrosive coating in two layers, but this method has a disadvantage of making the process complex.